Detecting Gases: What You Can't Smell Can Hurt You

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Walking through a drill-site area, a worker smells rotten eggs and stops tor a minute or two to assess where a gas leak might exist. Rubbing his itchv eves as he investigates further, he notices that he no longer smells the tell¬tale hydrogen sulphide odour. He does not realize that the gas has deadened his olfactory senses and unless he leaves the area within seconds, he could lose consciousness and possibly his life. A well-researched and planned gas-detection strategy can help reduce the likelihood of situations such as this.

This article covers the types of gas risks and the fixed-gas detection technologies and methods available to mitigate those risks. It also covers the safetv benefits of standards and approvals.

Mitigating gas risks

Risks from gas leaks come in many forms. Broadly defined, the categories are toxic gases, combustible gases, and gases that are both toxic and combustible. The risks of toxic gas leaks include death by asphyxiation or poisoning of the body. Combustible gas leaks pose risks such as fire or explosion after exposure to an ignition source, which could be merely the friction associated with gas escaping from a pipe fissure. An example of a gas that is both toxic and combustible is the deadly hydrogen sulphide gas.

One of the most effective ways to mitigate the risks associated with gas leaks is to follow good engineering and design practices during the design, planning and execution of processes and facility construction. Also important is to implement well-designed coverage into a gas-detection strategy.

An effective gas-detection strategy often includes both portable and fixed detection devices. Portable gas detectors are small and can be carried on a pocket or on a belt for personal protection. As the individual walks from area to area, the portable detector monitors the air for specific toxic or combustible gases. An alarm indicates that the individual might be walking through or standing in a potentially unsafe location. Fixed gas detection systems, on the other hand, are installed at fixed locations to provide long-term service life and detection for that area. The detector monitors a specific location all day. Addressed in this article are technologies and approaches that concern only fixed gas detection.

Fixed-detection techniques

Fixed gas detectors survey an area by using a variety of techniques. Gas-detector manufacturers offer the following types of fixed-gas-detection options: point, open-path, acoustic, and analytic or sampling detection systems.

Point gas detectors: Either combustible or toxic gas sensors can be fitted into point-type gas detectors. These detectors monitor a specific point or area and are strategically located for effective gas detection. In Urge critical areas, point detectors often are installed in a grid. Whether a gas is lighter or heavier than air determines where the detectors are mounted: high or low. These detectors require calibration for the gas type to be detected and, though some are self-monitoring, many must be routinely inspected to ensure they are capable of sensing gas.

Open-path or Hne-of-sight (LOS) gas detectors: Open-path gas detectors consist of a pair of modules that monitor the presence of combustible hydrocarbon gases within the infrared light beam projected between them. The gas/vapour hazard must pass through the light beam; therefore, the modules must be strategically located and properly aligned. Open-path detectors must be calibrated for the gas type to be detected and often are self-monitoring for a blocked light beam or electronic trouble.

Acoustic detectors: Ultrasonic gas-leak detectors sense the high-frequency sound emitted by high-pressure gas leaks. The detector can detect sounds of leaking gas independent of ambient conditions, such as wind direction. The gas leak must be released under pressurize. Ultrasonic gas leak detectors do better when they complement traditional gas detection methods because the ultrasonics do not detect specific gas types or values of Lower Lxplosive Limit (LEL) or toxic parts per million (ppm) concentration.

Analytic/sampling gas detection systems: Some analytical instruments extract an air sample, analyze the sample, and exhaust or return the sample to a safe location. These systems are mounted generally on a sub-plate within an enclosure with compression fittings for sample tubing connections.

Best fits for combustible gases

For combustible gases, the most common detection choices are catalytic and infrared point gas detectors.

Catalytic sensors detect a wide range of combustible gases both hydrocarbon and non-hvdrocarbon, such as hydrogen and acetylene. Catalytic sensors offer good repeatability and accuracy with fast response time and low initial cost. But at high combustible gas concentrations, there might be insufficient oxygen to sustain the catalyzing process, resulting in a false reading that understates the actual gas concentration. Catalytic sensors require routine calibration (typically every three months or less). Catalytic sensors are susceptible to poisoning from exposure to substances such as silicones, halogens, tetraethvl lead, acid, PVC vapours, and other corrosive materials. Sensors can fail without annunciation, hence the requirement tor routine calibration or bump testing.

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